Patch "pid_ns: Fix race between setns'ed fork() and zap_pid_ns_processes()" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

pid_ns: Fix race between setns'ed fork() and zap_pid_ns_processes()

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
pid_ns-fix-race-between-setns-ed-fork-and-zap_pid_ns_processes.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Kirill Tkhai <[email protected]>
Date: Fri, 12 May 2017 19:11:31 +0300
Subject: pid_ns: Fix race between setns'ed fork() and zap_pid_ns_processes()

From: Kirill Tkhai <[email protected]>

commit 3fd37226216620c1a468afa999739d5016fbc349 upstream.

Imagine we have a pid namespace and a task from its parent's pid_ns, which made setns() to the pid namespace. The task is doing fork(), while the pid namespace's child reaper is dying. We have the race between them:

Task from parent pid_ns             Child reaper
copy_process()                      ..
  alloc_pid()                       ..
  ..                                zap_pid_ns_processes()
  ..                                  disable_pid_allocation()
  ..                                  read_lock(&tasklist_lock)
  ..                                  iterate over pids in pid_ns
  ..                                    kill tasks linked to pids
  ..                                  read_unlock(&tasklist_lock)
  write_lock_irq(&tasklist_lock);   ..
  attach_pid(p, PIDTYPE_PID);       ..
  ..                                ..

So, just created task p won't receive SIGKILL signal, and the pid namespace will be in contradictory state. Only manual kill will help there, but does the userspace care about this? I suppose, the most users just inject a task into a pid namespace and wait a SIGCHLD from it.

The patch fixes the problem. It simply checks for (pid_ns->nr_hashed & PIDNS_HASH_ADDING) in copy_process(). We do it under the tasklist_lock, and can't skip PIDNS_HASH_ADDING as noted by Oleg:

"zap_pid_ns_processes() does disable_pid_allocation() and then takes tasklist_lock to kill the whole namespace. Given that copy_process() checks PIDNS_HASH_ADDING under write_lock(tasklist) they can't race;
if copy_process() takes this lock first, the new child will be killed, otherwise copy_process() can't miss
the change in ->nr_hashed."

If allocation is disabled, we just return -ENOMEM
like it's made for such cases in alloc_pid().

v2: Do not move disable_pid_allocation(), do not introduce a new variable in copy_process() and simplify the patch as suggested by Oleg Nesterov. Account the problem with double irq enabling found by Eric W. Biederman.

---
 kernel/fork.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

@@ -1590,11 +1590,13 @@ static struct task_struct *copy_process(
 	*/
 	recalc_sigpending();
 	if (signal_pending(current)) {
-		spin_unlock(&current->sighand->siglock);
-		write_unlock_irq(&tasklist_lock);
 		retval = -ERESTARTNOINTR;
 		goto bad_fork_cancel_cgroup;
 	}
+	if (unlikely(!(ns_of_pid(pid)->nr_hashed & PIDNS_HASH_ADDING))) {
+		retval = -ENOMEM;
+		goto bad_fork_cancel_cgroup;
+	}
 
 	if (likely(p->pid)) {
 		ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace);

@@ -1645,6 +1647,8 @@ static struct task_struct *copy_process(
 	return p;
 
 bad_fork_cancel_cgroup:
+	spin_unlock(&current->sighand->siglock);
+	write_unlock_irq(&tasklist_lock);
 	cgroup_cancel_fork(p, cgrp_ss_priv);
 bad_fork_free_pid:
 	threadgroup_change_end(current);

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/pid_ns-fix-race-between-setns-ed-fork-and-zap_pid_ns_processes.patch